The present invention generally relates to stepping motors, and more particularly to a stepping motor in which a core and a bearing are made from the same member.
One of the present inventors has previously proposed a flat type stepping motor in a Japanese Laid-Open Utility Model Publication No. 57-47886. This previously proposed stepping motor comprises a rotor having a disc shape, bearings for supporting a rotary shaft of the rotor, cores fitted over the outer peripheries of the respective bearings and having coils wound thereon, and stators each having inner and outer yokes. The rotor is magnetized in the thickness direction thereof, and has a plurality of magnetic poles arranged in the circumferential direction thereof with the different poles in alternation. The inner and outer yokes of each stator are arranged so that projections of the inner and outer yokes are separated from and oppose corresponding depressions of the outer and inner yokes, where both the inner and outer yokes have the projections and the depressions in alternation in radial direction. This previously proposed stepping motor uses oil impregnated bearings having non-magnetic copper as the base. The cores having coils wound thereon, are cylindrical and are made of a ferromagnetic material. These cores are fitted over the outer periphery of the bearings.
There is a general demand to downsize the flat type stepping motor. However, the cores must each have a cross sectional area to a certain extent so that the magnetization does not become saturated by the magnetic fields which are generated by the coils. Thus, the diameters of the cores which are fitted over the bearings cannot be reduced, and thee is a limit to the downsizing of the flat type stepping motor. Further, because the bearings and the cores are made of independent members, the number of parts is large and there is a problem in that the productivity is poor.